Carcass locations implicate cyanobacterial toxicity in a mass elephant mortality event in Botswana

Résumé

The 2020 mass mortality of 350 African elephants (Loxodonta africana) in Botswana sparked global concern. These deaths have been linked to cyanobacterial toxins (cyanotoxins) in local watering holes (pans), but evidence remains inconclusive. Our study presents the first detailed spatial analysis that explores the relationship between the ecohydrology of 3,389 regional pans with the locations of deceased elephants. Our findings reveal a significant difference in the distribution of fresh versus older elephant carcasses (p < 0.001), suggesting that the die-off event deviates from typical regional patterns of elephant deaths. We identified twenty pans near the sites of fresh carcasses that experienced more phytoplankton (microalgae or cyanobacteria) bloom events in 2020 (n = 123) compared to the previous 3 years combined (n = 23), exhibiting the highest average phytoplankton biomass of the period 2015-2023 (Normalised Difference Chlorophyll Index > 0.2; p < 0.001). These findings suggest a high risk and likelihood of cyanotoxins as the poisoning source. Our spatial analysis indicates elephants walked an average of 16.5 km (± 6.2 km) and died within 88 hours (± 33 hours) from initial exposure, offering metrics that were previously unknown for elephants. This study presents important new evidence that implicates cyanobacterial toxicity in the 2020 mass die-off and provides a general framework for investigation of future mortality events of large mammals. We underscore the need to integrate spatial analysis and ecohydrological assessments to better monitor and mitigate animal mortality events and inform conservation strategies.

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